The present invention relates to an artificial stereophonic circuit and an artificial stereophonic device which converts a monophonic input signal into a signal in a stereophonic format.
Most of a human voice frequency distribution concentrates in the vicinity of 300 Hz to 3.5 KHz. The vicinity of 1 KHz is important to the articulation of a conversation and a wavelength of 1 KHz is approximately 30 cm and a half wavelength is 15 cm.
Accordingly, when a voice having a frequency of 1 KHz arrives from the left in a transverse direction, it reaches a right ear in an opposite phase to a left ear because the right ear is distant by approximately 15 cm as compared with the left ear. More specifically, in the case in which the same sound having a frequency of 1 KHz arrives from the left and the right, a listener feels that an image of sound source is present on the front. In the case in which the phase of the sound on the right side is delayed by 180 degrees from the sound on the left side, the listener feels that an image of sound source is localized on the left side.
On the other hand, in the case in which a sound comes from the front, a voice frequency band (300 Hz to 3.5 KHz) is emphasized through an earlobe and an ear hole. When a sound comes from just the side, a frequency characteristic is almost flat.
A conventional artificial stereophonic circuit particularly reduces a sound volume in a voice frequency band (300 Hz to 3.5 KHz) in a frequency band of 20 Hz to 20 KHz than that in other frequency bands, thereby enhancing a stereophonic effect. Furthermore, a sense of sufficient spread cannot be obtained from only a sound volume difference. In the frequency band of 20 Hz to 20 KHz, therefore, a phase shift of 90 degrees to be a phase difference with which an image of sound source is not localized in a transverse direction and which can easily give a sense of spread is set between L and R channels.
FIG. 6 is a diagram showing the conventional artificial stereophonic circuit in consideration of such a respect. The reference numeral 21 denotes a monophonic signal input terminal, the reference numerals 22L, 23L and 24L denote a phase-shifting circuits for an L channel, and the reference numerals 22R, 23R and 24R denote a phase-shifting circuits for an R channel. The reference numeral 25 denotes a coordination (composite) circuit and includes an adder 26, a band-elimination filter (BEF) 27 and adders 28 and 29. The reference numeral 30 denotes an artificial L channel output terminal and the reference numeral 31 denotes an artificial R channel output terminal.
The three phase-shifting circuits 22L, 23L and 24L on the L channel side which are cascade connected have such a structure as to relatively and always maintain a phase difference of 90 degrees within a frequency band of 20 Hz to 20 KHz for the three phase-shifting circuits 22R, 23R and 24R on the R channel side which are cascade connected. In other words, a frequency band of 20 Hz to 20 KHz is divided into three bands and a phase difference of 90 degrees is relatively maintained through a phase circuit having pairs of 22L and 22R, 23L and 23R, and 24L and 24R for the bands. (See Bedrosian, S. D., “Normalized Design of 90 Phase-Difference Networks, “IRE Transactions on Circuit Theory, Vol. CT-7, June 1960).
An artificial stereophonic signal is generated by the coordination circuit 25 from the L signal and the R signal which have a phase difference of 90 degrees. First of all, the L signal and the R signal obtained by phase inversion are added in the adder 26 so that an L-R signal is generated and is inputted to the band-elimination filter 27. In the band-elimination filter 27, the level of a voice frequency band (300 Hz to 3.5 KHz) with which a sense of direction of a human ear is easy to understand is attenuated based on a frequency characteristic shown in FIG. 7, and a signal emphasizing a reverberation sound or an echo sound is fetched and is inputted to the adders 28 and 29. In the adder 28, the L-R signal is added to the L signal and is outputted to the L channel output terminal 30. In the adder 29, a signal obtained by inverting the phase of the L-R signal is added to the R signal having a phase difference of 90 degrees for the L signal and is outputted to the R channel output terminal 31.
As described above, the conventional artificial stereophonic circuit attenuates the level of the voice frequency band (300 Hz to 3.5 KHz) to cause a sound coming from the front to pretend to be a sound coming from the side. Furthermore, a sense of sufficient spread cannot be obtained from such a sound volume difference only. Therefore, three phase-shifting circuits are cascade connected to each channel of LR and a phase of 90 degrees to be a phase difference with which an image of sound source is not localized in a transverse direction and the sense of spread can be easily produced is added in a frequency band of 20 Hz to 20 KHz.
In the artificial stereophonic circuit shown in FIG. 6, however, changes in a phase and a sound volume become remarkable within a frequency band of 20 Hz to 20 KHz through the filter 27 for eliminating a component of a frequency band of 300 Hz to 3.5 KHz. Therefore, there is a problem that the localization of an image of sound source becomes unclear to obtain an unnatural sense of stereo.
Moreover, the circuits, for example, the phase-shifting circuits 22L, 23L, 24L, 22R, 23R and 24R, the filter 27 and the like are used. Therefore, a large number of (at least eight) capacitors are required, and furthermore, a great capacitance value is required for the capacitors. For this reason, it is necessary to externally attach the capacitors when wholly forming an IC. Consequently, there is a problem that the number of IC pins is increased. By using a gm amplifier having a high output impedance, it is possible to constitute a filter to be required for a capacitor having a low capacitance. However, it is impossible to avoid deterioration in S/N and a distortion factor.
Furthermore, in the case in which an interval between speakers is small, that is, approximately 20 cm or less, a sufficient stereo effect cannot be obtained.
In the artificial stereophonic circuit shown in FIG. 6, furthermore, two types of driving circuits are required for driving the speaker. In particular, in the case in which the conventional device is to be used for artificial stereophonic reproduction, it is necessary to additionally provide a circuit for the artificial stereophonic reproduction and a speaker, thereby a cost is increased.
It is a first object of the present invention to provide an artificial stereophonic circuit in which a change in a phase can be minimized to obtain a natural sense of stereo, and furthermore, the number of capacitors can be reduced.
It is a second object of the present invention to provide an artificial stereophonic circuit and an artificial stereophonic device in which a natural sense of stereo can be obtained by means of two speakers, and furthermore, a cost can be reduced without requiring the addition or modification of the circuit.
It is a third object of the present invention to provide an artificial stereophonic circuit and an artificial stereophonic device which can produce a stereophonic effect also in the case in which an interval between speakers is small, for example, 20 cm or less.